Generally speaking, a hydraulic excavator includes: a boom that is raised and lowered relative to a turning unit; an arm swingably coupled to the distal end of the boom; and a bucket swingably coupled to the distal end of the arm. A drive system installed in such a hydraulic excavator includes, for example, a boom cylinder driving the boom, an arm cylinder driving the arm, and a bucket cylinder driving the bucket. These hydraulic actuators are supplied with hydraulic oil from pumps via control valves.
For example, Patent Literature 1 discloses a hydraulic excavator drive system 100 as shown in FIG. 11. In the drive system 100, boom raising is performed by extension of boom cylinders 101 and 102, and arm pushing is performed by extension of an arm cylinder 103.
To be specific, the boom cylinder 101 and the boom cylinder 102 are connected to a boom first control valve 121 and a boom second control valve 122 by a boom raising supply line 123 and a boom lowering supply line 124. The arm cylinder 103 is connected to an arm first control valve 131 and an arm second control valve 132 by an arm pushing supply line 133 and an arm crowding supply line 134.
The boom first control valve 121 and the arm first control valve 131 are disposed on a first center bleed line 112, which extends from a first pump 111 to a tank. The boom second control valve 122 and the arm second control valve 132 are disposed on a second center bleed line 114, which extends from a second pump 113 to the tank.
At the time of boom lowering operation, the boom cylinders 101 and 102 are retracted by the weight of, for example, the boom. Therefore, at the time of boom lowering operation, it is desired to efficiently utilize the hydraulic oil discharged from the boom cylinders 101 and 102.
In this respect, in the drive system 100, the boom raising supply line 123 and the arm pushing supply line 133 are connected to each other by an acceleration line 140. The acceleration line 140 is provided with an acceleration valve 141. When a boom lowering operation and an arm pushing operation are performed concurrently, the acceleration valve 141 is opened, and thereby the operating speed of the arm cylinder 103 is increased.